The present invention relates to optical injection locking. More specifically the present invention relates to optical injection locking of a laser array to increase the power of a laser beam.
The generation of continuous, high optical output power from semiconductor lasers operating at room temperature is of great interest. For example, in the field of signal processing, a powerful laser beam is necessary for light to pass through an optical fiber having a length on the order of optical fibers lengths used in the telecommunication industry.
One approach that has succeeded in obtaining over a watt of output power per laser mirror facet is to coherently combine the power of many lasers operating on the same chip. Phase coupling of adjacent lasers is achieved by distributed coupling among the lasers of the array. (See "Experimental and Analytic studies of coupled stripe diode lasers", D. R. Scifres, W. Streifer, R. D. Burnham, IEEE J. Quantum Electron, Vol. QE-15, p. 917-922, Sept. 1979 for a full discussion of the design and construction of laser arrays.) Although some devices have exhibited single spectral mode and single far-field lobe emission, they are unpredictable. In general, the array outputs have been multimoded and double lobed. This can be understood by refernece to FIG. 1 which is a schematic representation of the spatial and spectral output of a laser array. Lines 10 represent the ends of several electrode stripes that comprise the laser array. In front of each stripe the intensity of light is greater than the areas between stripes. This is represented by pseudo-graph 12 which shows line 14 that represents the intensity of light as a function of position relative to base line 16 at the array facet. Note how the peaks of intensity 14a occur in front of the stripes 10.
The phase of the light emitted by the array is shown in pseudo-graph 18 of FIG. 1, which is a plot of the phase of the light as a function of position (line 20) relative to zero (line 22). As can be seen from line 20, the phase of the light as a function of position varies. The phase can have positive or negative values depending on position. The result is a far field intensity distribution, see FIG. 2a, that is multi-lobed. The far-field intensity distribution is the diffraction pattern of the optical field distribution at the array facet (see "Experimental and Analytic studies of coupled stripe diode lasers", D. R. Scifres, W. Streifer, R. D. Burnham, IEEE J. Quantum Electron, Vol. QE-15, p. 917-922, Sept. 1979). FIG. 2a is a graph of the far-field intensity of light as a function of far field angle. Peaks 30 and 32 are two unique and distinct lobes. By having two or more broad far-field lobes the laser light can not be collimated or focused, thus making it very difficult to transmit the light through an optical fiber, since the light cannot be focused to a small point that is required to feed the light into the fiber. Also, the array output spectral intensity distribution is multi-moded, See FIG. 2b. This is caused by random phase fluctuations in the light emitted by the array, see FIG. 1. This makes it impossible to transmit large bandwidths along the fiber since each mode propagates through the fiber at a different velocity resulting in different parts of a signal arriving at a destination point at different times. Deciphering the signal correctly would for all practical purposes be impossible. Furthermore, none of the uniform array structures reported thus far have operated in both a single spectral mode and a single far-field lobe.
The control of the phase and frequency of one discrete laser by another discrete laser by optical injection locking has been demonstrated and used for phase control, wavelength stability, microwave generation using FM sideband locking of the master laser, and locking together of multiple lasers. But in most of these cases, the laser diode being locked had a single spectral mode and a single lobe output, but being a discrete device, had only modest power of less than 10 mW.
It has been shown in "Chirped Arrays of Diode Lasers for Supermode Control", E. Kapon C. Lindsey, J. Katz, S. Margalit, and A. Yariv Appl. Phys. Lett. Vol 45 (30) Aug. 1, 1984, pgs 200-202; and "Phase Locked Injection Laser Arrays With Nonuniform Stripe Spacing", by D.E. Ackley Electron. Lett., Vol. 20, No. 17, Aug. 16, 1984, pgs. 695-697 Sthat single lobe operation can be accomplished by designing the array with nonuniform spacing between the lasing stripes. That alters the coupling between adjacent stripes and by proper design the coupling coefficients can be selected to produce array modes whose amplitude closely match the available gain profile of the laser structure. In this way, a single lobe spatial output can be obtained. But that technique does not assure single spectral mode operation, especially over all output levels of the diode array.